MIAMI — Atlanta Hartsfield became the first global airport to cross the century mark, processing its 100 millionth passenger of 2015 over this past weekend. The world’s busiest airport is projected to end the year having handled just over/under 101 million passengers, a truly historic total that represents sizzling 5.5% year over year (YOY) growth off of an enormous base.

Much of the growth at Atlanta has been due to the fortress hub of its largest operator, Delta Air Lines. Delta’s massive hub now numbers nearly 1000 flights per day, with just under 78% of those occurring on mainline aircraft according to masflight. Delta carried roughly 79.2 million passengers at Atlanta in 2014 and will likely cross 83 million in 2015 when the final figures are tallied early next month.

Meanwhile overall traffic growth at Atlanta has come despite a sharp reduction in traffic for its second biggest tenant, Southwest Airlines. Since the AirTran merger, Southwest has steadily drawn down the once powerful Atlanta hub, shifting to an operation mostly focused on point to point (p2p) flights aimed at origin and destination (O&D travelers).

From a peak of just under 14 million in 2012, Southwest is projected to carry fewer than 10 million passengers at Atlanta in 2015. Even though ultra-low cost carrier Spirit has filled in some of that gap, Atlanta’s overall traffic figure is only made more impressive by the decline of its second most important airline.

Atlanta is the undisputed (heavyweight?) airport traffic champion of the world

The world’s busiest airports rankings have undergone wild swings over the past decade. But one thing has remained remarkably consistent – Atlanta’s utter and complete hegemony atop the rankings of the world’s busiest airports by passenger traffic.

Back in 2005, Atlanta processed roughly 85.9 million passengers, 9.4 million more than its nearest competitor, Chicago O’Hare which was itself 8.6 million passengers ahead of London Heathrow. Then,  the recession came. As Chicago O’Hare’s traffic fell off in the face of infrastructure challenges and the developed world struggled with the effects of the late 2000s recession, a resilient Atlanta buoyed by the rise of low cost carrier AirTran Airways found itself 22 million passengers clear of its nearest competitor (London Heathrow) in 2009 by a margin of 88 million to 66 million.

Traffic at O’Hare had fallen off a cliff to 64.1 million, the lowest number since 1993. And Asia had risen. Beijing was all of a sudden the world’s third busiest airport, shooting up to a massive 65.4 million passengers from 24.2 million passengers and a spot outside the top 30 in 2001. For the moment, Beijing and Tokyo Haneda were still the only two Asian airports in the top 10 (with Hong Kong and Dubai joining them in the top 15).

Fast forward to 2014, and the rise of Asia was complete. Beijing was now the world’s second busiest airport with 86.1 million passengers (trailing Atlanta by just 10.1 million), effectively quadrupling in traffic since 2001. Asian airports were half the top 15 and top 25, and particularly notable was the rise of Middle Eastern hubs Dubai and Istanbul, which hit 70.5 million and 56.9 million passengers respectively. Both airports displayed even steeper growth curves than Beijing, as in 2002, passenger traffic was 16.0 million and 11.4 million respectively.

The world’s busiest airport race is no longer a foregone conclusion

Then a funny thing happened in 2015. Beijing’s growth slowed down as a murky Chinese economy mixed with ATC challenges to keep growth under 3%. Dubai and Istanbul were more resilient, but a consistent theme in each of the three megacities was that growth in airport traffic was not that much higher than at Atlanta (6-8% in Dubai/Istanbul and <3% n). In fact, it is likely that the gap between Atlanta and Beijing will actually grow in 2015 (Beijing is likely to come in under 90 million). Dubai and Istanbul may inch closer, but they are so far behind Atlanta that they are several years of 6-8% growth away from catching up.

And the trends in these global megacities point away from them ever catching up to Atlanta, at least on an airport specific basis. Each of these cities has instead moved towards and embraced a multi-airport approach, similar to that of London, Tokyo, or New York. In Beijing and Istanbul, global megacities of 21.5 million and 14.4 million people respectively, this is justified by geography and population alone.

Istanbul has already seen much of its recent traffic growth shift to Sabiha Gokcen International Airport, which is on the Asian side of the intercontinental city (the larger Ataturk International Airport is on the European side). Istanbul is rapidly constructing a new international airport 22 miles north of the city in the unobstructed countryside, scheduled to open in 2018 or 2019, which will replace Ataturk.

But the new airport will be inconvenient to Istanbul’s city center, and while Turkish Airlines will have no choice but to shift its global hub to the new airport, some domestic travel may actually shift to Sabiha Gocken, which also has room to expand and is planned to remain open even after the new airport emerges. The first phase of the new international airport will only handle 90 million passengers, and the split airport system may keep the larger of Istanbul’s airport

Meanwhile Beijing is planning the development of the new Beijing Daxing International Airport 29 miles south of the city, but it plans to keep Capital Airport (the current hub) open and operational. As was witnessed with Shanghai when it opened Pudong International Airport, new growth may shift to Daxing Airport, but it will take several years for the new airport to build up its traffic base, years during which Atlanta can continue to put distance between itself and its nearest competitors. And some carriers will prefer to remain at the existing airport, driving down the growth potential of Daxing.

Dubai is in a similar situation with Dubai World Central, where all carriers except Emirates will likely shift over the next few years. Emirates will stay in the current Dubai International Airport till 2023 (perhaps longer), and the maintenance of split operations between the two airports means that it will be several years until a Dubai airport catches up to Atlanta.

The consistent theme in all three cities is that despite their total airport system traffic surpassing that of Atlanta (as London, New York, Tokyo, and Paris already have), the division of that traffic amongst multiple airports will continue to keep Atlanta in pole position as the world’s busiest airport for years, perhaps more than a decade to come.